Apparatus for spray drying milk or the like

ABSTRACT

An apparatus for spray drying milk in which drying hot air is blown against a spray of milk sprayed from a nozzle disposed in the upper portion of a drying tower, and at the same time cooled air is introduced into the drying tower so as to surround the flow of hot air so that the portions of the inner walls of the tower which would otherwise be overheated are covered with a layer of cooled air. Further, a first spray of milk in the form of a generally diverging conical sheet is provided in the tower, and simultaneously a second spray is applied to the first spray from its outside in such a manner that the particles of the second spray impinge upon the particles of the first spray at a transient position between a constant-rate drying period and a falling rate drying period of the particles of the first spray, whereby the particles of the second spray are combined with those of the first spray.

llnited Males Patent [72] Inventors lKatsuto Okada 1,964,858 7/1934 Peebles 159/4 S Tokyo; 2,324,526 7/1943 Morgenthaler 99/71 lFumio Kata, Kawasaki-slid, both of Japan 2,842,193 7/1958 Ballestra 159/4 D [21] Appl. No. 880,163 3,126,289 3/1964 Spilman et a1. 99/203 [22] Filed Nov. 26, 1969 Palemed Nov- 971 :nr rary LEamrner-Yrbufr L. Bascomb, Jr

73] Assignee Morinaga Milk industry Co Ltd mmmer 0 er AuorneyKurt Kelman Tokyo, Japan [32] Priority Feb. 1, 1969 1 331mm ABSTRACT: An apparatus for spray drying milk in which dry- [31] 414/7085 ing hot air is blown against a spray ofmilk sprayed from a nozzle disposed in the upper portion of a drying tower, and at the same time cooled air is introduced into the drying tower so as [54] RATUS FOR SPRAY DRYING MILK THE to surround the flow of hot air so that the portions of the inner 3 CW 6 D in walls of the tower which would otherwise be overheated are 3 Figs covered with a layer of cooled air. Further, a first spray of milk [52] 111.8. 1C1 159/4, in the form of a generally diverging conical sheet is provided l59/DIG. 23, 99/206 in the tower, and simultaneously a second spray is applied to [51] int. C11 B01d 1/16 the first spray from its outside in such a manner that the parti- [50] Field 01 Search 159/4, 4 B, cles of the second spray impinge upon the particles of the first 4 A, 4 D, 4 S, 4 R, 2, 4 C, 4 D, 4 E, 48, D16. 23; spray at a transient position between a constant-rate drying 99/203, 206; 1 17/100A period and a falling rate drying period of the particles of the first spray, whereby the particles of the second spray are com- References cued bined with those of the first spray.

UNITED STATES PATENTS 1,830,174 11/1931 Peebles 159/4 t 1 6]? 1, 1 1, I 11 s Z7 Q i515 W r i v/Ifllilllllii'nll- PATENTEDuuv 23 I97! SHEET 3 [IF 3 FINE CONICAL SHEET SPRAY INVENTORS KATS U T'o OMDA Ill BY F0 M l0 KA'T APPARATUS FOR SPRAY DRYING MILK OR THE LIKE In the manufacture of powder products, such as powdered mil, hot air type spray drying apparatus in which the particles of sprayed liquid are dried by hot air have been widely used. The hot air type spray drying apparatus according to the prior art has the disadvantage that some of the particles of sprayed liquid stick to the periphery of the blow-in port of a duct for feeding the hot air as well as to the inner walls of the drying tower in their half-dried or dried-up state, which particles grow to form lumped particles, and then these lumped particles or burned portions thereof drop downwards so as to be mixed into the product, thus deteriorating the quality of the final product. This is particularly true when the spray drying apparatus is operated continuously for a long time. In order to overcome this problem of mixing of burned particles into the product, and attempt has been made to apply cooling air onto the outer walls of the drying tower so as to cool the tower from the outside. This attempt, however, has a drawback in that, if sufiicient cooling is effected, the temperature of the air adjacent the inner walls of the tower reaches the dew point. On the other hand, if the cooling temperature is low enough to prevent formation of drops of dew on the inner wall of the tower, the desired amount of cooling can not be obtained.

Furthermore, the spray drying apparatus according to the prior art has the disadvantage that it can not produce particles of uniform size. In order to remove this disadvantage, two attempts have been made, one being to spray finely divided particles against a spray immediately upon the emission thereof, and the other being to spray finely divided particles against the spray from its inside. In the former method, the finely divided particles are blown directly onto the particles sprayed by a nozzle, so the spray itself is disturbed and therefore the sizes of the particles can not be made uniform. In the latter method, a distributor for the finely divided particles and its pipe arrangement must be provided in the drying tower, so half-dried particles and dried up particles of the sprayed liquid stick to the surfaces of the distributor and the pipe arrangement where they grow into larger sized particles which eventually drop downwards to be mixed into the product, thus rendering the product useless.

Such known spray drying apparatus is not able to manufacture continuously and efficiently powder products of uniform size and excluding heterogenous components, such as burned particles.

SUMMARY OF THE INVENTION An object of the present invention is to provide an apparatus for spray drying milk or the like which is capable of mass-producing continuously powder products, particularly powdered milk, of high quality.

Another object of the present invention is to provide an apparatus for spray drying milk or the like which is capable of manufacturing continuously powder products not containing degenerated components, such as burned particles.

A still further object of the present invention is to provide a method of spray drying milk or the like and an apparatus therefor which is capable of manufacturing powder products of uniform size.

Yet another object of the present invention is to provide a method of spray drying milk or the like and an apparatus therefor which is adapted to coat the particles of sprayed liquid to be dried.

According to the invention, an apparatus for spray drying milk or the like is provided which includes means for supplying cooledand dehumidified air into a drying chamber or tower along the inner wall of a pipe or duct for supplying drying hot air thereto. Thus, the layers of the air adjacent to the inner wall of the hot air supply duct and the inner walls of the upper portion of the drying tower subjected to heating by the hot air are replaced by cooled air, so the particles of sprayed liquid are never caused to stick to the duct and to these wall portions, due to adhesion caused by molten fat, electrostatic force, etc., even if the particles include some fat components which have melting points nearly equal to room temperature, whereby burned particles or particles degenerated by overheating are prevented from being mixed into the final product.

Further, the invention provides a method of spray drying milk or the like and an apparatus therefor in which a first spray of milk or the like is provided by means of a pressure or so-called swirl nozzle and in which a second spray of finely divided particles of powdered milk or other components, such as fat, is applied to the first spray from its outside in such a manner that the particles of the second spray impinge upon the particles of the first spray at a transient position between a constant rate drying period and a falling rate drying period of the particles of the first spray. Thus, when the second spray is made of the same component as that of the first spray the sizes of the particles finally manufactured are made uniform, and when a second spray is made of the different component from that of the first spray, the particles of the first spray are coated so that the final product is of a desired nature.

Other objects and advantages of the present invention will become apparent with reference to the following description of the invention made in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an embodiment of the apparatus according to the invention in vertical sectional view.

FIGS. 2a to 2e are horizontal cross sectional views at various indicated cross sections of the apparatus shown in FIG. 1.

FIG. 3 shows diagrammatically the temperature distributions at various cross sections of the apparatus shown in FIG. 1.

FIG. 4 shows diagrammatically the distributions of air velocity at the same cross sections of the apparatus of FIG. I as taken in FIG. 3.

FIG. 5 shows a modified form of the apparatus shown in FIG. 1.

FIG. 6a shows a modification of the apparatus according to the invention.

FIG. 6b shows the arrangement of FIG. 6a incorporated in the upper part of the drying tower of FIG. 1.

FIGS. 7a and 7b are horizontal cross sectional views taken along lines aa and 12-h in FIG. 6alooking upward.

FIG. 8a shows a modified form of the apparatus shown in FIG. 6.

FIG. 8b shows the arrangement of FIG. 8a incorporated in the upper part of the drying tower of FIG. 1.

FIG. 9 is a sectional view taken along line aa in FIG. 8a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. I there is shown the main part of an embodiment of the apparatus according to the invention in vertical sectional view, and in FIGS. 2a-2e are shown the cross sectional views respectively at cross section na, b-b, c-c, d-d and ee of the apparatus of FIG. I for the purpose of facilitating the understanding of the construction thereof. Concentrated milk F is fed through a feed pipe 102 to an atomizer 104, such as a pressure swirl nozzle, rotary disk, or a two fluid nozzle, by which it is sprayed so as to produce a spray I03 in the form of a diverging conical sheet in a drying chamber or tower 101. Hot air H supplied through a conduit 105 is blown against the spray 103 so as to evaporate the moisture in the particles of sprayed milk. On the other hand, cooled an dehumidified air C fed through a conduit 108 is emitted from an orifice 109 so that an annular film of cooled air is formed on the inner wall of hot air supply duct I06 and so that the area surrounding the spray I03 in the drying tower is filled with cooled air. In the region within the upper part of the drying tower, a resilient boundary wall is formed between the cooled and the hot air so that the cooled air layer surrounds the hot air. In other words, that said resilient boundary wall, the ceiling ll] of the drying tower and the inner sidewall 110 of the drying tower define a space to which the cooled air is admitted. The hot air H is introduced tangentially to the tower side wall 110 (FIG. 2a) and then its rotational velocity component is changed to a vertical velocity component so that the hot air H has a predetermined velocitydistribution. The velocity of the hot air at any point on a concentric circle at a cross section (e.g. -b, cc, etc. in FIG. 1) of the duct 106 is equal and constant, despite the fact that there is only one blow-in port for the hot air. The amount of the cooled air C is so small, that if the diameter of the conduit I08 is relatively large, the cooled air has the same velocity at any point on a concentric circle after it is discharged from the orifice 109, since the dynamic pressure of the cooled air can be neglected even if it is introduced tangentially to the tower sidewall 110. Of course, if the velocity ofthe hot air H is so small that its dynamic pressure at the time of introduction can be neglected, it is easy to control the hot air flow so that is has the same vertical velocity at any point in a horizontal plane. The amounts of the hot air H and the cooled air C may be controlled by automatic valve means (not shown) respectively connected to the conduits 105 and 108. The amount of the concentrated milk F may also be controlled by valve means (not shown) connected to the feed pipe 102. The relative temperature distributions at cross sections b-b, c-c, g-g of the apparatus of FIG. 1 are shown graphically by the curves h, g of FIG. 3, respectively. The relative air velocity distributions at cross sections b-b, cc,...., g-g of the apparatus of FIG. 1 are shown graphically in FIG. 4, respectively. In FIG. 4, the farther below the base lines a point on the curves is, the higher is the velocity of the airflow represented by the point, and any point on the curves positioned above the base lines represents the occurrence of counterflow of the air. As seen from FIGS. 3 and 4, the periphery of the inner wall of the duct 106 and the ceiling 111 as well as inner sidewall of the drying tower are not exposed directly to the hot air.

In the prior art hot air type spray drying apparatus of this type, particles oflarge size would be ejected from the atomizer 104 and deposit on the inner sidewall 110 at the time of starting or stopping the apparatus, for example, or even during the normal running state. The fine particles of the sprayed milk would deposit on the duct 106, particularly the blow off point 107, and on the ceiling 111 as well as the inner side wall 110 of the drying tower 101, but in the apparatus of FIG. 1 such deposition is almost avoided since parts 110, 106, 107 and 111 are exposed to the cooled air, and even if such particles have been deposited on these parts, neither formation of burned particles or thermally degenerated particles nor growth of the deposited particles occurs. The dried up powder product is discharged from an output port (not shown) provided at the lower end of the tower.

The amount of the cooled air is extremely small in comparison to that of the drying hot air and the temperature ofthe hot air drops by 2 to 3 C. at most upon contact with the coaled air so that this temperature drop exerts substantially no influence on the drying process. Further, the flow of the cooled air along the inner wall of the duct 106 serves not only to cool the wall but also to control the hot air flowing through the duct so as to prevent counterflow of the air from occurring. Further, since cooled and dehumidified air is used, the inner wall of the drying tower bears no drop ofdew even when the apparatus is stopped after a relatively long running thereof.

It has been found that more etficient cooling can be attained by providing another orifice 112 at the position of the ceiling 111 adjacent to the wall 110, as shown in FIG. 5, and that it is preferable to cause both the hot and cooled airs blown into the tower from the duct 106 to swirl slowly Because centrifugal force acts on the respective air layers so that the outer cooled air having larger specific gravity does not tend to be mixed with the inner hot air having smaller specific gravity. The air is exhausted from the tower by means of air exhausting means (not shown) connected to the tower.

FIG. 6a shows a part of a modification of the apparatus according to the invention in vertical sectional view. The arrangement of FIG. 6a may be incorporated in the upper part of the drying tower of FIG. 1 as shown in FIG. 6b. In this case, cool air introduced into the drying tower may serve to cool the portions within the drying tower which would be overheated if it were not for the supply of cool air, as in the arrangement of FIG. 1. But, for the sake of clarity, the description will be made with respect to FIG. 60 rather than FIG. 6b. In FIGS. and 7b are shown the horizontal cross-sectional views respectively at cross section a-a and b-b of the apparatus of FIG. 6a, looking upward, for the purpose of facilitating the understanding of the construction thereof. Concentrated milk F is fed through a feed pipe 602 to an atomizer 604 by which it is sprayed so as to produce a spray 603 in the form ofa generally conical sheet. It is preferable to use a pressure or swirl nozzle for the atomizer 604. Hot air H is blown through conduit 605 against the spray 603 so as to evaporate the moisture in the particles of sprayed milk. Upon ejection from the atomizer 604, the particles spread radially and downwards so as to form said spray 603 by virtue of surface tension, and when the particles react with the hot air H, the particles lose almost all their moisture to form particles almost dried but including an extremely small amount of moisture, whereupon they fall vertically downwards.

It has been recognized (The Fundamentals of Food Engineering, by Stanley E. Charm, The Avi Publishing Company. Inc. Westport, Conn., 1963) that, in drying powder particles, the rate of drying or the rate of removal of moisture from the powder particles is constant for a period of time and the con' stant-rate drying period is followed by a falling rate drying period during which the drying rate steadily decreases. In accordance with the invention, very fine particles D of the same material as that contained in the liquid to be dried (which have been prepared by collecting and selecting, for instance by a cyclone or bag filter, from a batch of previously spraydried particles) are fed through another feed pipe 613 to fine particle supply means 614 whence they are ejected into the drying tower so that they impinge upon the particles of the first spray 603 at a transient position between the constantrate drying period and the falling rate drying period of the particles of spray 603, at which position they are combined with these particles. In FIG. 6a, the transient position between the constant rate drying period of the particles of spray 602 is indicated at A. Among the particles forming the spray 603, the number of particles having a relatively small size is much larger than the number of particles having a relatively large size and, because of the fact that they are sprayed from a swirl nozzle, the particles having a relatively small size are distributed at the outer portion of the hollow conical spray 603 while the particles having a relatively large size are distributed at the inner portion of the hollow conical spray 603. Therefore, the probability that the fine particles D are combined with the spray particles having a relatively small size is much larger than the probability that the fine particles D are combined with the sprayed particles having a relatively large size. Thus, the sizes of the particles produced are made uniform without disturbing the spray 603. The flow of the especially fine particles D is indicated by 615 in FIG. 6. It is desirable to supply a small amount of cooled air in the region indicated by 618. The fine particles D may be permitted to impinge upon the particles of the spray 603 at the position A by employing baffles 617 and 616, each having a proper angle with respect to the vertical, which are disposed at the lower end of the fine particle supply means 614, or by controlling the amount of air which carries and conveys the fine particles D. To this end, baffles of various angles may be used interchangeably. Whether or not the fine particles D have impinged properly upon the particles of spray 603 at the transient position A may be ascertained by examining the product obtained at the bottom of the drying tower, the degree of uniformity of the product particles being at a maximum when the fine particles D impinge upon the particles of spray 603 properly at the transient position A. When the distance between the atomizer 604 and the position A is long, a baffle 617, having a relatively large angle with respect to the vertical may be used or the amount of the air for carrying the fine particles D may be large so as to increase the distance of travel of the fine particles D from the baffle 617 to the position A, and when the distance between the atomizer 604 and the position A is short, baffle 617 having a relatively small angle with respect to the vertical may be used or said amount of the air for carrying the fine particle may be small so as to decrease said distance of travel of the fine particles D. The span angle of the spray 603 is substantially constant over the whole area of the flow if a pressure type swirling nozzle having a predetermined configuration is used.

FIG. 8a illustrates another type of fine particle supply means 814 having a plurality of pipes adapted to eject the fine particles D (See to FIG. 9 showing a cross sectional view along line aa of FIG. 8) and the fine particles D fed through a feed pipe 813 and ejected from pipes 814 impinge upon the particles of a spray 803 at a transient position A between a constant rate drying period and a falling rate drying period of the particles of the spray 803. An extremely small amount of air is introduced by filter means (not shown) through gaps G for the purpose of preventing the components of the drying tower from being overheated by the hot air H. In FIG. 8a the same component is indicated by the same reference numeral as that used in FIG. 6 except that 6" appearing at the IOOs place in each reference numeral used in FIG. 6a is replaced by 8 in FIG. 8a.

In the arrangement of FIG. 8a, if atomizers are fixed to the tips of the fine-particle supply pipes 814 and if fine particles of different material from that of the liquid to be dried, such as fats, are sprayed into the drying tower so that the fine particles impinge upon the particles of the spray 803 at the position A, coating of the particles of the spray 803 may be effected and thus the nature of the powder product obtained may be changed.

In FIGS. 6a and 8a, when the fine particles D are ejected into the drying tower, they are exposed to the hot air instantaneously for a certain time period, but since this time period is so short and comparable to the sterilization time at most, they are not degenerated by heat, but a desirable result is obtained from the sanitary standpoint.

Since only one atomizer (604 or 804) is employed, control, operation and conservation of the spray drying apparatus are easy in comparison to a case when a plurality of atomizers are employed, and therefore a stable spray can be generated in the drying tower.

Examples of atomizers applicable to the spray drying apparatus of FIGS. 6 and 8 are a centrifugal atomizer, a two fluid nozzle and a pressure or swirl nozzle. When a pressure or swirl nozzle is employed, it is preferable to increase the velocity of the liquid at the inlet of the vortex chamber (which results in decrease in the apparent viscosity of the liquid) and to eject the liquid from an orifice which is slightly smaller than the vortex chamber. When a pressure type swirl nozzle is used in this preferred way, a slightly larger power is required than in a case when a plurality of small nozzles of another type is used for obtaining particles of the same size as that of the particles produced by the pressure type swirl nozzle, but the size of the nozzle can be as small as that of one such small nozzle, whereby the standard deviation of the particle distribution of the spray generated by the nozzle is smaller than that of the spray generated by the plurality of small nozzles without controlling the nozzle. Also the particles of larger size are gathered in the center portion and the particles of smaller size at the outside portion of the spray when the spray pressure is more than about 100 kg./cm. so that the amount of recirculated fine particles is decreased. Actual practice has shown that a pressure type swirl nozzle having a vortex chamber of l5 mm. diameter and an orifice of 9-13 mm. diameter treated satisfactorily concentrated milk having a viscosity of 60 cp. at

a rate of5,000 l.(hr. under a spraypressure of 200 kg./cm.

Although the illustrated embodiments of the present invention have been described referring, by way of examples, to the manufacture of dry milk or powder milk, it is to be understood that the present invention is also applicable to the manufacture of other powder products. It will be evident to those skilled in the art that the method and apparatus according to the present invention are not limited to those described hereinbefore and many modifications or alterations may be made without departing from the scope and spirit of the present invention.

What is claimed is:

1. In a spray drying apparatus comprising a drying tower having a vertical axis and an interior wall coaxial therewith, the drying tower having an upper part; atomizer means mounted coaxially in the upper drying tower part for spraying a liquid medium containing a solid into the tower; a hot air feeding means mounted in the upper drying tower part and including a hot air inlet duct having an elongated coaxial wall ending in an upper annular edge and disposed substantially coaxially within the tower for supplying hot drying air downwardly into the tower; and a cool air feeding means mounted in the upper drying tower part, the atomizer means being located below the cool air feeding means: the improvement of the cool air feeding means including a duct having an annular edge lying in the plane of the upper annular edge of the hot air inlet duct, the annular edge of the cool air feeding means being concentric with and radially inward of the annular edge of the hot air inlet duct, and disposed to define an annular cool air inlet orifice with the annular edge of the hot air inlet duct along the wall thereof, and a source of cool and dehumidified air connected to the cool air feeding means for supplying a thin, annular film of cool and dehumidified air through the annular cool air inlet orifice along the inner surface of the hot air inlet duct wall so as to surround the hot air therein, the annular film of cool air also flowing along the interior wall of the tower.

2. In the spray drying apparatus of claim I, the atomizer means being arranged to provide a first spray of the liquid medium containing a solid in the form of a generally conical sheet; and means mounted in the upper drying tower part for applying a second spray of fine particles of the solid to the conical surface of the first spray, the second spray applying means being disposed to impinge the fine particles upon the first spray at a transient position between the constant rate drying period and the falling rate drying period of the first spray.

3. In the spray drying apparatus of claim 2, the second spray applying means comprising a vertically disposed coaxial feed tube having an upper end communicating with a source of the fine particles and a converging lower end defining an output aperture, the atomizer means comprising a coaxial pipe supplying the liquid medium and passing through the feed tube and having an end below the output aperture thereof, and a diverging baffle means connected to the pipe and defining a delivery channel for the fine particles with the converging lower end of the feed tube, the bafile means directing the fine particles to the transient position.

4. In the spray drying apparatus of claim 2, the second spray applying means comprising a plurality of circumferentially equidistantly spaced feed pipes each extending generally downwardly with a slight tilt angle inwardly relative to the vertical axis of the tower, the pipes having upper ends communicating with a source of the fine particles and being so arranged that the particles emitted from the pipes impinge upon the first spray at a transient position between the constant-rate drying period and the falling rate drying period of the first spray. 

2. In the spray drying apparatus of claim 1, the atomizer means being arranged to provide a first spray of the liquid medium containing a solid in the form of a generally conical sheet; and means mounted in the upper drying tower part for applying a second spray of fine particles of the solid to the conical surface of the first spray, the second spray applying means being disposed to impinge the fine particles upon the first spray at a transient position between the constant rate drying period and the falling rate drying period of the first spray.
 3. In the spray drying apparatus of claim 2, the second spray applying means comprising a vertically disposed coaxial feed tube having an upper end communicating with a source of the fine particles and a converging lower end defining an output aperture, the atomizer means comprising a coaxial pipe supplying the liquid medium and passing through the feed tube and having an end below the output aperture thereof, and a diverging baffle means connected to the pipe and defining a delivery channel for the fine particles with the converging lower end of the feed tube, the baffle means directing the fine particles to the transient position.
 4. In the spray drying apparatus of claim 2, the second spray applying means comprising a plurality of circumferentially equidistantly spaced feed pipes each extending generally downwardly with a slight tilt angle inwardly relative to the vertical axis of the tower, the pipes having upper ends communicating with a source of the fine particles and being so arranged that the particles emitted from the pipes impinge upon the first spray at a transient position between the constant-rate drying period and the falling rate drying period of the first spray. 